Fuels



2,993,766 FUELS Benjamin Thomas Fowler, Abingdon, England, assignor to Esso Research and Engineering Company, a corporation of Delaware No Drawing. Filed Jan. 7, 1959, Ser. No. 785,331 Claims priority, application Great Britain Jan. 13, 1958 6 Claims. (Cl. 44-70) The present invention is concerned with improved hydrocarbon fuels of the type suitable for use in aviation gas turbines, which have a reduced tendency to deposit carbonaceous matter at elevated temperatures.

Aviation gas turbines presently use refined hydrocarbon fuels boiling in the gasoline and/or kerosine range, for instance, boiling within the range 100 to 600 F. particularly from 300 to 600 F. Typical specifications include a Reid vapour pressure of about 2 to 3 lbs. per square inch and a freezing point between 75 to 40 F. These values however may be more or less to meet specific operating conditions. The hydrocarbon fractions are sweetened fractions obtained by the substantial removal of sulphur, e.g. by extractive sweetening using caustic solution. It is particularly important that the total sulphur content of the fuels be as low as possible, for instance, below 0.4% by weight, and the mercaptan sulphur to below 0.001% by weight. The presence of even small proportions of sulphur can cause the deposition of gum deposits when exposed to temperatures of the order of 500 F., presumably by oxidation with oxygen dissolved in the fuel. In the past fuel temperatures in the aviation gas turbine engine have been sufficiently low to avoid the formation of harmful deposits in the precombustion stage of the fuel system. However, high performance axial flow engines have, by virtue of high compressor pressure ratios, relatively low fuel consumption, and the use of the fuel as a heat sink increases fuel temperatures so much that in some, heavy carbonaceous deposits may be formed which interfere with the normal fuel distribution.

The present invention is based on the discovery that the tendency of aviation gas turbine fuels to deposit carbonaceous matter at elevated temperatures may effectively be inhibited by the presence of naphthenic acids in proportions above 0.1% based on the total fraction. The naphthenic acids are preferably present in proportions ranging from 0.1% to 0.3% by weight. In these instances where the fuel fraction itself contains naphthenic acids, for instance fuels derived from naphthenic containing crudes such as Gulf or Californian crudes, or South American crudes, such as Tia Juana, the naphthenic acids present in the crude exert a deposit-inhibiting effect, and therefore fractions derived from such naphthenic crudes are especially useful as aviation gas turbine fuels, providing the fractions contain a sufficient proportion of naphthenic acids.

If a naphthenic-containing fraction is contacted with alkaline treating agents, for instance when extractive sweetening is carried out using alkaline solutions such as caustic soda solutions, the naturally-occurring naphthenic acids are removed along with mercaptans in the form of naphthanates and mercaptides respectively. This results in a substantial increase in the deposit-forming tendency at elevated temperatures. This effect is shown by the following experiment, wherein a hydrocarbon fraction derived from a Tia Juana crude, and having a boiling point range from 334 to 600 F. was designated Oil A and a similar fraction treated for 5 minutes with by volume of a 10 Be caustic soda solution was designated Oil $3.!

The two oils A and B were then subjected to OFR fuel coker ratings at two temperature levels.

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Fuel (Joker Conditions Merit rating Merit rating Oil A 800 770 Oil B Q 870 110 Thus it will be seen that the untreated product substantially maintained its merit rating at the higher temperatures, whereas the caustic extracts oil gave a very low' merit rating at the higher temperatures. Further the preheater tube deposit level obtained after running five hours on Oil A was little more than that obtained in one hour on Oil B.

Thus a preferred embodiment of this invention involves recovering the naphthenic acids from the caustic extract from a naphthenic hydrocarbon fraction, and incorporating the recovered naphthenic acids into the extracted fraction so that the extracted fraction contains between 0.1% and 0.3% of naphthenic acids, based on the total fraction.

The recovery of the naphthenic acids from the caustic extract may be achieved by well-known chemical methods, for instance, by neutralizing the caustic extract with a mineral acid, such as hydrochloric acid, and extracting the naphthenic acids with an extraction agent, e.g. ether.

The present invention may be illustrated by the following example, wherein an additive was prepared from the caustic extract derived from Oil B described above. This extract was neutralized with hydrochloric acid and extracted with petroleum ether. 129 grams of a brown viscous liquid were obtained in this way from 3.5 litres of caustic liquor. This extract termed extract A was blended back into Oil C, an extracted fraction similar to Oil B and further CFR fuel coker tests carried out. The following results were obtained.

CFR coker test merit rating at 400 to 500 F.

It will be seen from the above results that an extracted oil containing less than 0.1% by weight of extracted acids showed little improvement in deposit-forming tendency, whereas the presence of more than 0.1% of acids resulted in a very substantial improvement.

What is claimed is:

1. A process for producing an aviation gas turbine fuel composition of improved thermal stability, which process comprises:

treating a hydrocarbon fraction derived from a naphthenic crude oil and boiling in the range between 100 and 600 F. and having less than 0.4% by weight sulfur and a freezing point between F. and 40 F., with a caustic soda solution to extract mercaptans to below 0.001 by weight based on the total fraction;

recovering the liquid naphthenic acid product from the caustic extract; and

incorporating an amount of the total liquid naphthenic acid product recovered into the extracted hydrocarbon fraction so that the extracted fraction contains between 0.1 and 0.3% by weight of naphthenic acids.

2. An aviation gas turbine fuel composition of improved thermal stability, which consists of a hydrocarbon fuel boiling within the range between 100 and 600 F. having less than 0.4% by weight sulfur, a freezing point between 75 F. and 40 F, and to which has been added between 0.1 and 0.3 by weight of the liquid naphthenic acid product obtained by treating a hydrocarbon fraction derived from a naphthenic crude oil and boiling in the range between 100 and 600 F. with a caustic soda solution to extract mercaptans to below 0.001 by weight based on the total hydrocarbon fraction, and recovering the saidliquid naphthenic acid product from the caustic extract.

3. A fuel composition as defined by claim 2 wherein said hydrocarbon fuel has a boiling range between 300 F. and 600 F.

4. A fuel composition as defined by claim 2 wherein said hydrocarbon fuel has a Reid vapor pressure of about 2 to 3 p.s.i.g.

5. A fuel composition as defined by claim 2 wherein said hydrocarbon fuel has a mercaptan sulfur content of less than 0.001% by weight.

6. An aviation turbine fuel composition of improved thermal stability, which consists of a hydrocarbon fuel boiling in the range between 300 and 600 F. having less than 0.4% by weight sulfur, a freezing point between F. and 40 F., a mercaptan sulfur content of less than 0.001 and to which has been added between 0.1 and 0.3% by weight of a liquid naphthenic acid extraction product obtained by treating a hydrocarbon fraction derived from a naphthenic crude oil and boiling in the range between and 600 F. with a caustic soda solution to extract mercaptans to below 0.001% by weight based on the total hydrocarbon fraction, separating the caustic extract from the hydrocarbon fraction, neutralizing said caustic extract with a mineral acid, and recovering the said liquid naphthenic acid extraction product with an extraction agent.

References Cited in the file of this patent Anderson et al. Apr. 2, 1946 I 

1. A PROCESS FOR PRODUCING AN AVIATION GAS TURBINE FUEL COMPOSITION OF IMPROVED THERMAL STABILITY, WHICH PROCESS COMPRISES: TREATING A HYDROCARBON FRACTION DERIVED FROM A NAPHTHENIC CRUDE OIL AND BOILING IN THE RANGE BETWEEN 100 AND 600*F. AND HAVING LESS THAN 0.4% BY WEIGHT SULFUR AND A FREEZING POINT BETWEEN -75*G. AND -40*F., WITH A CAUSTIC SODA SOLUTION TO EXTRACT MERCAPTANS TO BELOW 0.001% BY WEIGHT BASED ON THE TOTAL FRACTION, RECOVERING THE LIQUID NAPHTHENIC ACID PRODUCT FROM THE CAUSTIC EXTRACT, AND INCORPORATING AN AMOUNT OF THE TOTAL LIQUID NAPHTHENIC ACID PRODUCT RECOVERED INTO THE EXTRACTED HYDROCARBON FRACTION SO THAT THE EXTRACTED FRACTION CONTAINS BETWEEN 0.1 AND 0.3% BY WEIGHT OF NAPHTHENIC ACIDS. 